1. Field of the Invention
The invention relates to removal of solids from liquid and, more particularly, to a process for removing of solids, particularly silica, from spent geothermal brine.
2. Description of the Prior Art
In view of rising costs for fossil fuels and the realization that supplies of such fuels are exhaustible, increasing attention has been focused on the earth's heat as a source of energy. Brine in subterranean rock formations or aquifers under certain geological conditions can reach high temperatures. The geothermal energy in this brine can be productively utilized, and it has been estimated that geothermal resources in the United States alone could produce 140,000 megawatts of power over a life expectancy of 30 years. This is enough energy to meet the needs of about 140 million people in the United States at current levels of electrical power consumption.
In one known process for the production of electrical energy from geothermal sources, brine under pressure is delivered from geothermal wells at temperatures ranging from 400.degree.-700.degree. F. and pressures of 200 -450 psia to a flash tank in which steam is flashed from the brine. The flashed steam is used to run steam turbines to generate electrical energy. A major problem associated with this process is the disposal of the resulting spent brine.
Spent geothermal brines are characterized by high concentrations of dissolved solids, including high concentrations of silica (SiO.sub.2). For example, in the Imperial Valley of California, the brines in the Niland Basin have total dissolved solids concentrations of up to 220,000 ppm (parts per million) and those in the Heber Region have total dissolved solids concentrations of aproximately 20,000 ppm. In comparison, seawater generally has a total dissolved solids concentrations of about 30,000 ppm. Both types of brines from the Imperial Valley have high silica concentrations in the range at 500-700 ppm, and geothermal brines from other geographical regions also contain high concentrations of silica.
In the subterranean geothermal aquifers, the geothermal brine is at temperatures ranging up to about 700.degree. F., and the dissolved solids are at equilibrium in the brine. However, after flashing the brine to atmospheric pressure and lower temperatures (about 200.degree. F. to about 230.degree. F.) to produce steam, the spent brine is supersaturated with silica. For example, the equilibrium concentration of silica in brine at 200.degree. F. is about 180 ppm; since the actual concentration can be as high as 700 ppm, the brine is often supersaturated. The presence of silica in this supersaturated condition results in the formation of scale, containing primarily silica, in the wells and related equipment thereby hindering satisfactory disposal of the spent brine.
According to one disposal method the spent brine is pumped through reinjection wells which return the spent brine to the same aquifers from which the hot brine is removed. This method is often preferred over other disposal techniques because it is both cost-effective and environmentally sound. It is cost-effective in that it reduces overall disposal costs and at the same time increases the production life of the geothermal source. It is environmentally sound in that large areas of land need not be dedicated to unsightly brine storage ponds, potential subsidence of agricultural land is prevented, and the release of odors often associated with geothermal waters is controlled.
However, because of the extreme scale-forming characteristics of the spent brine, deposited solids must be periodically removed from the reinjection wells. This results in both increased downtime and increased operating costs. In order to prevent such solids deposition, spent brine must be treated prior to reinjection to produce a stable solution. That is, the concentration of dissolved silica in the brine which is to be reinjected must be reduced to at or below its saturation level at equilibrium.
One proposed method of reducing the dissolved silica concentration is spent geothermal brine is to add ammonium hydroxide to the brine to raise its pH. This method is taught in U.S. Pat. No. 4,016,075. According to the patent the hot brine is treated with ammonium hydroxide in a reaction chamber and then transferred to flash tanks. The ammonium hydroxide raises the pH of the brine and reacts chemically with dissolved aluminum and ferrous ions to from a gelatinous sludge of aluminum and ferrous hydroxides; silica is captured in the gelatinous sludge. From the flash tanks the spent brine is transferred to a settling tank wherein the sludge is allowed to settle from the brine and the silica thereby removed. Part of the resulting sludge is transferred to disposal, and part can be returned to the reaction chamber. According to the patent, the ammonium hydroxide is added to raise the pH of the brine from its normal pH of about 5.0 to a pH preferably about 6.6 to form the gelatinous sludge. Also according to the patent this pH adjustment requires the added ammonium hydroxide solution to be about 1.5 percent by volume of the total brine solution.